Objective:We tested the hypothesis that phase-delayed circadian rhythms underlie seasonal affective disorder (SAD) by measuring phase position of 6-sulfatoxymelatonin excretion and comparing antidepressant response to morning or evening light given as a first treatment.

Design:Randomized controlled trial.

Setting:Ambulatory.

Patients:Thirty-two women and seven men with SAD.

Intervention:Light therapy (2500 lux for 1 hour for 1 week) was administered either at 7 AM or 10 PM, preceded by a baseline week and followed by a withdrawal week.

Results:Our SAD patient sample was moderately depressed (Hamilton Depression Scale [HAM-D] score 18); a HAM-D reduction of 50% or more was found in 12 of 18 patients given morning and in 15 of 21 patients given evening light (70% response rate). Response was not dependent on age, gender, stage of the menstrual cycle, time of year, or on the timing or duration of sleep. Urinary 6-sulfatoxymelatonin was measured in 30 patients; 22 had phase-delayed circadian rhythms. However, phase position was correlated neither with depth of depression nor with a preferential response to morning or evening light.

Comment:Both morning and evening light therapy improved depressive symptoms in patients with SAD independent of their circadian phase or sleep timing. These findings argue against a circadian phase-delay hypothesis of the pathophysiology of SAD, or the necessity of a phaseadvance by morning light for clinical efficacy. They additionally suggest more practicable and flexible schedules for light therapy in SAD, since time of day is not crucial.